Response of Populus euphratica Oliv.sap flow to environmental variables for a desert riparian forest in the Heihe River Basin,Northwest China

Being an important desert riparian forest in the lower reaches of the Heihe River Basin,Populus euphratica Oliv.forest functions as a natural barrier in maintaining and preserving the stability of local oases.Accordingly,accurately estimating the water use of P.euphratica is important for vegetation protection and water resource allocation.To date,little data are available for evaluating the hysteretic effects between sap flow and environmental variables,and for estimating the water use of desert riparian forest.In this study,we measured the sap flow velocity(Vs)of P.euphratica using the heat ratio method during the growing season of 2012.Based on the response of Vsto solar radiation(Rs)and vapor pressure deficit(VPD),we estimated the hourly Vsand daily Vsusing the multivariable linear regression and a modified Jarvis-Stewart(JS)model,respectively.Hysteretic response of Vsto environmental variables was then evaluated using a sap flow model.We found the thresholds of Vsresponses to Rsand VPD at both hourly and daily scales during the growing season,and successfully estimated the seasonal variations of hourly Vsand daily Vsusing the JS model.At an hourly scale,the maximum Vsoccurred earlier than the maximum VPD by approximately 0.5 h but later than the maximum Rsby approximate 1.0 h.At a daily scale,the maximum Vslagged behind the maximum VPD by approximately 2.5 h while occurred earlier than the maximum Rsby approximately 2 h.However,hysteretic response of Vswas weakened when Rsand VPD were measured together using the JS model at both hourly and daily scales.Consequently,short-term and intensive field campaigns,where Vsand environmental variables can be measured,may be used to estimate short-run sap flow and stand transpiration using only two environmental variables.